To measure power and/or power factors at different measuring points in a.c. voltage networks, in particular at feed-in points and branches, for example of a switching station, appropriate measuring instruments, e.g., instruments for measuring network quality, are configured at these locations, these instruments usually having voltage transformer inputs and current transformer inputs. Current transformers, which are connected at the current transformer inputs of the measuring instrument there, are provided for the individual phases, at the measuring (monitoring) points. The voltage transformer inputs of the measuring instruments arranged at the various measuring points are connected via lines to voltage transformers, which are arranged at a central voltage detecting site in the a.c. voltage network, e.g., in the bus bars in a switching station. The transmission of voltages from the voltage detecting site to the individual measuring instruments at the measuring points is influenced, particularly in the case of large switching stations, by interference in the form of noise, injection of external signals, and voltage displacements caused by ground-fault currents. In addition, the outlay for circuit engineering is very high, because of the necessity for the voltage transformer inputs at each measuring instrument and the lines for transmitting voltage from the voltage transformers at the voltage detection site to the individual measuring instruments. This applies correspondingly when voltage transformers are provided for each phase, instead of at each individual measuring point.
An object of the present invention is to reduce the circuit engineering outlay needed for measuring power and/or power factors at a plurality of measuring points in an a.c. voltage network.
The object is achieved in accordance with the present invention by providing a measuring system having at least one measuring instrument for measuring power and/or power factors at least one measuring point in an a c. voltage network, a voltage transformer device having in each case one voltage transformer for each phase of the a c. voltage network being arranged at a voltage detecting site of the a.c. voltage network. In addition, a current transformer for each phase is mounted at each measuring point and connected at a current transformer input of a measuring instrument allocated to the particular measuring point. The voltage transformer device and all measuring instruments are interconnected via a shared data transmission device, in which predefined information on the detected voltages is transmitted digitally to the measuring instruments.
One essential advantage of the measuring system according to the present invention, with respect to its measuring instruments, is that in contrast to equivalent measuring instruments under the state of the art, the need is eliminated in each case for voltage transformer inputs and their requisite downstream electronics and associated cabling. Instead, the information pertaining to the voltages of the a.c. voltage network required for measuring power and/or power factors, is transmitted digitally from the voltage-detection site to the individual measuring instruments. In this context, the information can be transmitted via an internal bus, a fiber-optic network, or as a wireless communication.
The digitally transmitted , predefined information on the detected voltages preferably includes the voltage values and the period duration. In this context, the detection time of the network period is determined very precisely to enable the power and/or power factors to be measured with high precision in the individual measuring instruments. Moreover, the measuring instruments is very precisely time-synchronized to avoid phase errors. To compensate for phase deviations between the voltage transformers and the current transformers at the various measuring points, the phase deviation is determined for each measuring instrument in a calibration operation, the phase-angle error preferably being input into a memory of the measuring instrument; on the basis of the stored phase-angle error, the measuring instrument automatically compensates the phase-angle deviation. To determine the phase-angle error, square-wave signals, for example, are fed to the measuring instrument via the current transformer inputs and the data-transmission device, the phase-angle error being defined in terms of the digitally transmitted voltage information, over the harmonic component.
Preferably configured at the voltage-detection site is a further measuring instrument having voltage transformer inputs, where the voltage transformers are connected, the additional measuring instrument being designed to generate the predefined information on the detected voltages. Thus, everywhere in the measuring system, the same measuring instruments are used, which are advantageously distinguished from conventional measuring instruments in that one of the measuring instruments, namely that at the voltage-detection site, does not have any current-transformer inputs, and the remaining measuring instruments do not have any voltage transformer inputs.